SDR Pictures

 I captured a set of pictures of radio signals today, 17th May 2017 at BH23 8DU.

The first is of the IoW beacon run by G8MBU at IO90IR. The frequency is near 1296.8MHz and the aerial I used to receive the beacon is a broadband discone at around 15 feet and fed with about 30 feet of coax. The receiver is an SDR Play. In this and the next picture I tuned the receiver to 1296.933 to avoid covering the beacon signal.


 The second picture is the same set-up but using a Lime SDR. The centre spike is a spurious response and the wideband input was used.


Now for some Long Wave reception. 

This picture shows the Lime SDR tuned to 198KHz. A long wire aerial is being used at the "modified" L input. I couldn't resolve a signal using either the W or H input.

 Now the same set-up but using my upconverter which shifts the band 0-30MHz to 50-80MHz; hence 50.198MHz = 198KHz

Mixed up with the various broadcasts is lots of local interference and some software artifacts etc. Read further to see more about this.
 Below is the same set-up but with the aerial input fitted with a 50 ohm termination. The left signal is 50MHz breakthrough from the crystal used for the mixer. The centre signal is a spurious response equivalent to that shown in the second picture showing beacon reception. Care needs to be taken because of this spurious response, for example if Auto-Centre is selected and employed, any station resting on top of this spike suffers from distortion. The amplitude of the spike changes when the gain settings are modified, but it always stays in the centre of the display.
 Finally, the SDR Play tuned to 198KHz using the long wire aerial and the IF gain backed off to -55dB to avoid overloading. You can clearly see the pair of VLF timing signals at 60 and 77KHz. There's lots of noise from switching power supplies. For example, the wide peak at 50KHz is from my desklamp which uses a low energy bulb.

 Many thanks go to Simon Brown in Cornwall for his hard work in producing the excellent program used in this exercise.

 The next thing I decided to do was to investigate the noise and spikes seen on the pictures from my SDR Play. The display can be set to one of several bandwidths and I'd noticed different noise signals at different bandwidth settings. To minimise external signals including locally generated noise entering the aerial socket I fitted a 50 ohm terminating plug here. This enabled me to see noise and signals emanating either from the innards of the SDR or perhaps via the USB cable. One can also see any artifacts being generated by software.

First the bandwidth set to 10MHz. A software artifact at the screen centre. An "artifact" is a posh word for "thing".

 Next the bandwidth reduced to 8MHz. Identical types of picture were seen also for bandwidths of 7, 6 & 4MHz.

 Now the bandwidth is set to 3MHz and some lumpiness can be seen at the sides.

 Now reduced to 2.048MHz and some spikes can also be seen.

 Down to 1MHz and the layout of spikes is different

 Finally in this sequence the bandwidth is set to 500KHz. In all the above tests I made sure the Zoom feature was set to maximum. Ordinarily one would adjust Zoom to show only the band you want to check, and you would also ensure the signal you wanted to receive was in an area away from the artifacts. A good example is that any signal must be moved to one side of any centre artifact otherwise you'll probably hear distortion on recovered audio.
 Now for some external interference.. first I fitted a small whip aerial to the SDR Play and held it near to the computer monitor. This particular noise of course is pretty strong because the aerial is close to the monitor, but the spikes are visible on most pictures, albeit at a lower level.

 Now, the same interference source but looked at in the LF/MF bands.
 Below, moving the test aerial away from the monitor reveals a completely different type of interference. This is coming from my local area network CAT5 cabling. It's mainly from a pair of IP cameras and explains the underlying shape of the picture above.

 Turning off the power to one of the two cameras results in the picture below. The lumpiness that remains is similar noise from the second IP camera which uses a remotely located switch and whose wiring is chiefly away from the computer.

 To show you what effect a low energy lamp has on reception, I moved the test aerial near to my desk lamp and turned it on.

This uses an Osram de-luxe Long Life 20W lamp.

 in progress

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